The need for energy storage and its rising demand has become a major concern that the world faces today and going forward into the future. Importantly, electrical grid storage in a range of scaled sizes will be increasingly necessary as green technologies such as wind power, and solar energy conversion become more prevalent. Presently, the options for electrical grid storage consist of pumped-storage hydro, compressed air storage, advanced flywheels, thermal sinks, and flow batteries. All these technologies have their merits and limitations. Also, conventional batteries, such as Pb-acid, Ni—Cd, and high-temperature sodium batteries of the type Na—S and Na—NiCl2, which use a molten sodium anode and operate at about 300-500° C., have also been utilized. These technologies all have their shortcomings, and perhaps the greatest is in terms of up-front capital and engineering costs. For example, the high-temperature batteries need to have expensive thermal management engineered components. In contrast, ambient temperature sodium batteries are a relatively new energy storage system that has been increasingly interesting and attractive due to the promise of low cost due to the great abundance of sodium. Abundant sodium would also provide an alternative chemistry to lithium batteries, and may therefore alleviate concerns regarding limited lithium reserves in the world [1].
The science and engineering of Li-ion batteries in various energy storage applications are quite developed. In contrast, the body of literature and the knowledge of basic and applied research on ambient temperature sodium and sodium-ion batteries are lacking New sodium and sodium-ion battery materials that can satisfy the need for energy storage and its rising demand, and which provide an alternative to lithium are difficult to find. In a sodium electrochemical system, perhaps the greatest technical hurdle to overcome is the lack of a high-performance cathode material that is easy to synthesize, safe, non-toxic, and low cost. The opportunity in this area is to develop a new cathode material that meets all of these criteria. Development of a new cathode would help enable the use of an ambient temperature sodium and sodium-ion battery on a large scale for electrical grid storage. In addition, if the cost and safety are suitable, then transportation application, and other energy storage applications can be utilized, particularly if the energy density of the sodium and sodium-ion battery can be improved.
Ambient temperature Na and Na-ion batteries are at their infancy, so new materials to enable Na and Na-ion electrochemistry and new redox couples could be numerous. Non-aqueous electrolyte ambient temperature sodium and sodium-ion batteries have electrochemical similarity to lithium and Li-ion batteries. While the voltage of Na/Na+ is only about 300 mV less positive than Li/Li+, the Na molecular weight of about 23 g/mol is much higher than that of Li (about 7 g/mol). Therefore new materials for Na and Na-ion batteries must have a desirable suite of attractive qualities to justify their implementation.
Some examples of sodium cathode materials are sodium vanadium phosphate fluoride-type material, NaVPO4F, used in sodium-ion cells [2,3], and lithium sodium vanadium phosphate fluoride [4] used in mixed lithium and sodium containing electrolytes, as well as Na2MPO4F (M=Fe, Mn) materials [5]. An interesting Na battery study using NaCrO2 was recently reported [6]. Unique reversible Cr(III/IV) oxidation state changes were observed. However, from a practical and environmental standpoint, NaCrO2 is difficult to handle and Cr(VI) is considered toxic.
Other exotic materials such as Ni3S2 [7], (MoO2)2P2O7 [8], and Fe3F—C materials [9] have been tested in sodium batteries, but these do not perform as well as desired and are difficult to process. Sodium vanadium oxides (e.g., NaV3O8) are also known in electrochemical Na cells [10], but the use of vanadium is not favorable due to toxicity concerns. Shiratusuchi et al. has suggested the use of FePO4 in sodium-ion (Na-ion) cells [11], but data on only two cycles and at 350° C. was provided. Lu and Dahn previously reported that P2-layered Na2/3[Ni1/3Mn2/3]O2 with space group (S.G.) P63mmc could reversibly exchange Na in sodium cells [12], but the voltage profile was complicated, showing single and two phase regions, and conversion to O2 structure [12] at high states of charge. In addition, these materials were difficult to synthesize, e.g., requiring temperatures as high as 900° C. with liquid nitrogen quenching. For Mn-only layered oxide compounds, examples include earlier works on NaxMnO2 [13], Na0.6MnO2 and Na0.75MnO2, [14]. It appears that a phase change to amorphous character and a loss of capacity occurs during cycling of such materials [15].
Rechargeable ambient temperature sodium and sodium-ion cells and batteries can be used for many energy storage applications, particularly electrical grid storage technologies. Other applications are possible for ambient temperature sodium and sodium-ion batteries including, but not limited to portable consumer products, tools, medical products, defense products, transportation, and aerospace products and other energy storage devices. There is an ongoing need for new cathode materials for sodium electrochemical cells and batteries. The present invention addresses this need.